Elderly people feeling drowsy during the day due to poor sleep or waking up in the night had a greater build-up in their brain of amyloid plaques — which consume the brain, kill cells, and eventually lead to total memory loss. And this makes sense: recent studies have shown that while the brain sleeps, it clears away deposits of amyloid.

Even though previous research showed a correlation between sleepiness and AD, scientists wondered if the accumulation of amyloid plaques in the patients’ brains caused sleep problems or if it was the other way around.

Now, a team led by Mayo Clinic’s Prashanthi Vemuri has cast light on the subject: sleep itself seems to be causing the plaque accumulation that triggers the neurodegenerative disease.

“This study is the first in humans to demonstrate a predictive association between a measure of sleep disturbance at baseline and change in an AD [Alzheimer’s disease] biomarker across multiple points,” Joseph R. Winer and Bryce A. Mander, of the University of California, wrote in an editorial published with the study in the Journal of the American Medical Association.

The research team studied 283 people aged 70 or older without dementia from the center’s Study of Aging. Each participant completed surveys that assessed their general sleepiness and had at least two consecutive imaging scans of their brains from 2009 to 2016. The scans monitored the difference in amyloid plaque quantity between two scans in different regions of the brain.

Researchers discovered that 63 participants (22.3%) had excessive daytime sleepiness, and this was associated with increased amyloid plaque accumulation in susceptible regions of the brain.

“We found that daytime sleepiness was causing more deposition of amyloid in people who are already amyloid positive, so it was influencing the rate of deposition over time,” Dr. Vermuri said.

Although the study seems to establish causation between sleep patterns and amyloid accumulation, the team still has no definitive answer to why and how sleep has this effect.

However, experts consider this study a breakthrough, believing the findings underline the importance of good sleep for preserving brain health.

“This study and others present evidence that poor sleep quality may be an early warning sign of AD-related processes,” Winer and Mander wrote. “Although a better understanding of the role of sleep in the AD cascade could soon lead to effective sleep-based therapies, at present, maintaining healthy sleep and treating clinical sleep disorders must be a current priority for mental health in older adults.: